1. Field of the Invention
The present invention relates to a light-emitting device and more particularly to a light-emitting device using a thin film transistor formed over a transparent substrate such as glass or plastic. In addition, the invention relates to an electronic equipment using the light-emitting device.
2. Description of the Related Art
In recent years, with the advance of the communication technology, mobile phones have been widely used. In the future, transmission of moving images and a larger volume of information is expected. On the other hand, through reduction in weight of personal computers, those adapted for mobile communication have been produced. Information equipment called PDA (Personal Digital Assistant) originated in electronic notebooks has also been produced in large quantities and widely used. In addition, with the development of display devices and the like, the majority of portable information equipment are equipped with a flat panel display, and a television set using a flat panel display has been taken the place of a conventional CRT television set.
Moreover, according to recent technologies, an active matrix display device tends to be used as a display device for the above electronic equipment.
In the active matrix display device, a thin film transistor (hereafter a TFT) is arranged in each pixel and a display screen is controlled by the TFT. Compared to a passive matrix display device, such an active matrix display device has advantages in that it achieves high definition and high image quality, and it can be used for moving images. Therefore, it is expected that the mainstream of display devices for portable information equipment will shift from a passive matrix type to an active matrix type.
FIG. 12 illustrates a configuration of a pixel portion for an active matrix light-emitting device. In each pixel, a gate electrode of a switching TFT 1201 is connected to a gate signal line (G1 to Gy) for inputting selection signals from a gate signal line driver circuit. One of a source region and a drain region of the switching TFT 1201 is connected to a source signal line (S1 to Sx) for inputting signals from a source signal line driver circuit whereas the other thereof is connected to a gate electrode of a TFT 1202 for driving a light-emitting element 1204 and one electrode of a capacitor 1203. The other electrode of the capacitor 1203 is connected to a power source supply line (V1 to Vx). One of a source region and a drain region of the TFT 1202 for driving the light-emitting element 1204 is connected to the power source supply line whereas the other thereof is connected to one electrode of the light-emitting element 1204.
The light-emitting element 1204 has an anode, a cathode, and a light-emitting layer provided between the anode and the cathode. In the case where the anode of the light-emitting element 1204 is connected to the source or drain region of the TFT 1202 for driving the light-emitting element 1204, the anode corresponds to a pixel electrode whereas the cathode corresponds to a counter electrode. Instead, in the case where the cathode of the light-emitting element 1204 is connected to the source or drain region of the TFT 1202 for driving the light-emitting element 1204, the cathode corresponds to a pixel electrode whereas the anode corresponds to a counter electrode.
Note that a potential of the counter electrode is called a counter potential and a power source for providing the counter potential to the counter electrode is called a counter power source in this specification. A potential difference between the pixel electrode and the counter electrode corresponds to a drive voltage, which is applied to the light-emitting element 1204.
In such a configuration of a pixel, the amount of current flowing into a light-emitting element is easily varied depending on characteristic variations of TFTs and the characteristic variations of TFTs directly leads to display variations. Thus, a current programming method has been developed, in which a signal current is inputted to a pixel instead of a signal voltage (e.g. see Document 1).
FIG. 6 illustrates a conventional current-input type pixel using a current programming method. Description is made on FIG. 6 below. The pixel shown in FIG. 6 comprises a source signal line 601, a gate signal line 602, a power source supply line 610, switching TFTs 603 and 604, a current-voltage conversion TFT 605, a voltage-current conversion TFT 606, a storage capacitor 607, a pixel electrode 608, and a light-emitting element 609.
An operation thereof is described below. In a current programming, the gate signal line 602 is selected to turn ON the switching TFTs 603 and 604. When the switching TFTs 603 and 604 are turned ON, signal currents are supplied from the source signal line 601 through the switching TFTs 603 and 604 to charge the current-voltage conversion TFT 605, a gate terminal of the voltage-current conversion TFT 606, and the storage capacitor 607. Consequently, the current-voltage conversion TFT 605 and the voltage-current conversion TFT 606 are both turned ON, and currents flow from drain terminals to source terminals thereof. The currents flow to the light-emitting element 609 through the pixel electrode 608.
Subsequently, in a non-current programming, the gate signal line 602 is not selected to turn OFF the switching TFTs 603 and 604. Accordingly, the drain terminal of the current-voltage conversion TFT 605 is in the floating state, therefore, no current flows to the current-voltage conversion TFT 605. However, the gate terminal of the voltage-current conversion TFT 606 has the potential stored by the storage capacitor 607 and currents are kept flowing to the voltage-current conversion TFT 606. Consequently, the light-emitting element 609 keeps emitting light.
When the current-voltage conversion TFT 605 and the voltage-current conversion TFT 606 have uniform characteristics, the same amount of current flows into the respective TFTs. Therefore, display variations as is in the conventional one shown in FIG. 12 do not occur easily (see Document 2).
[Document 1]
Japanese Patent Application Laid-Open No. 2001-147659.
[Document 2]
Japanese Patent Application Laid-Open No. 2003-162254.
In such a pixel configuration, however, in a current programming, a current enough larger than a current during light emission must be supplied from source signal lines to a pixel portion. The reason is that the source signal line has large parasitic capacitance and the parasitic capacitance must be charged and discharged until a necessary potential is obtained.
Therefore, a current flows through the source signal line 601, the switching TFT 603, the current-voltage conversion TFT 605, and the light-emitting element 609 in this order in a current programming. Accordingly, the light-emitting element 609 emits light by the current during the current programming. This light emission results in the light emission that is not proper light emission after the current programming, and luminance that is not proper required luminance occurs, thus an accurate gray scale has not been achieved.